1. Field
This patent specification relates to a communication terminal and facsimile apparatus, and more particularly to a communication terminal and facsimile apparatus connected to an analog communication network and provided with a power management feature.
2. Background
With the rapid growth in data communication, including the Internet, small portable communication terminals have came into wide use. When a communication terminal is connected to an analog communication network, such as a public switched telephone network (PSTN), electrical isolation between the network and the internal circuitry of the terminal is needed for protecting against damage to the circuitry in the terminal caused by surge voltage, penetration of common mode noise, etc. A relatively massive isolation transformer has been used for such purposes for a long time.
Such an isolation transformer should convert an input voltage to an output voltage without distortion of a signal waveform. However, in practice factors such as distortion of an electromagnetic characteristic of the iron core of the transformer, can distort the output voltage. Further, as an isolation transformer is miniaturized, such distortion tends to increase.
Further, there has been a rapid increase in data transmission rates in analog communication networks. Consequently, there is a greater need to reduce distortion of a transmission signal in order to achieve higher data rate transmission. That is, the need has been growing for further reduction in distortion of an analog transmission signal, and the need for further miniaturization of an isolation transformer also has been increasing.
More recently, isolation in the analog signal region is being supplanted by isolation technology in a digital signal region, in parallel with rapid progress of semiconductor and insulating material technology. In digital isolation technology, an analog signal received from an analog communication network is first converted into a digital signal at a line interface circuit, and the converted digital signal is used for electrical isolation in the digital signal region. Similarly, transmitting digital data is isolated in the digital signal region, and the isolated digital data is converted into an analog signal and then transmitted to the analog communication network.
As an example, Japanese Laid-Open Patent Application No. 6-98038 discusses a modem with digital isolation that eliminates the need for a relatively large and heavy isolation transformer from a data access arrangement (DAA) and substitutes two relatively small pulse transformers between an integrated analog circuit and a digital signal processor to eliminate analog distortion.
When electrical isolation is achieved in a digital signal region, allowable signal distortion is larger than that achieved in an analog signal region, and therefore a relatively small isolation device, such as a pulse transformer, a photocoupler, etc., can be used. As another example of a small isolating device in a digital signal region, an isolation circuit having a combination of electric capacitors and semiconductor devices is also used. Isolation in a digital signal region eliminates the need for a large and expensive isolation transformer, thereby reducing the size and cost of a communication terminal connected to an analog communication network.
Another pertinent consideration is to reduce the power consumption of a communication terminal, such as a facsimile apparatus. For example, to reduce power consumption, a facsimile apparatus may provide a power-saving state in which electric circuits other than a monitoring circuit are placed in a waiting state or idling state. In the waiting state, the monitoring circuit monitors an incoming call received from the connected network and a manual instruction input by an operator of the apparatus.
In a communication terminal providing digital isolation, a modulated analog communication signal received from a connected network is first converted into a digital signal by a line interface circuit as described earlier. The converted digital signal is transmitted to a digital signal processing circuit, such as a digital signal processor (DSP), after passing through a digital isolation interface circuit. Through processing by the digital signal processing circuit, the transmitted digital signal is demodulated, i.e., reconstituted into original data, i.e., data before being modulated at a sending terminal, and the demodulated data is transmitted to a next stage.
Conversely, transmitting data received from a preceding stage is modulated by the digital signal processing circuit, and the modulated data is transmitted to the line interface circuit after passing through the digital isolation interface circuit. Then the modulated digital data is converted into an analog signal as a modulated communication signal, and the modulated communication signal is sent out toward a receiving terminal via the connected analog communication network.
The above described digital signal processing circuit also receives network monitoring signals, such as an incoming call detection signal, an inverting polarity detection signal, a busy tone detection signal, etc., from the line interface circuit via the digital isolation interface circuit. The digital signal processing circuit informs a following processor, such as a main controller, of the received monitoring signals. Further, the digital signal processing circuit responds to the received monitoring signals with an acknowledgment signal, as necessarily.
Accordingly, the digital signal processing circuit needs to be supplied with electrical power and in operation for receiving the network monitoring signals even in a power-saving state during a waiting state or an idling state of the communication terminal.
When the digital signal processing circuit is in operation, the digital signal processing circuit consumes a relatively large quantity of electric power due to a high speed digital signal processing capability executed by large scale circuitry for modulating and demodulating operations at a modulation and demodulation section.
Consequently, in communication terminals including facsimile apparatuses having isolation in a digital signal region, a power-saving function is not always fully effective because substantial amount of power is consumed by the digital signal processing circuit in a power-saving mode at a waiting state of the communication terminal.